Insulating coating layer for electronic circuit board, insulating coating material for electronic circuit board, and method for forming insulating coating layer for electronic circuit board

ABSTRACT

The present invention relates to an insulating coating material for an electronic circuit board, an insulating coating layer for an electronic circuit board, and a method for forming such insulating coating layer, and the insulating coating material is a solution comprising a thermoplastic resin and a solvent containing a low boiling solvent, which solution can be prevented from dropping after its application onto the electronic circuit board, and the insulating coating layer is formed using the coating material on the surface of the electronic circuit board having edge portions thereon so that the coating thickness at the edge portions and that at the other portion than the edge portions are almost equal to each other, further, the method for forming such insulating coating layer is characterized by forming the coating layer using the above insulating coating material according to a spray method or a dipping method.

DETAILED DESCRIPTION OF THE INVENTION Industrial Utilization Field

The present invention relates to an insulating coating layer for anelectronic circuit board with elements mounted thereon and a method forforming same. More particularly, the present invention is concerned withan improvement of a coated state of edge portions havingelectroconductivity on the board.

Moreover, the present invention relates to an insulating coatingmaterial for an electronic circuit board and more particularly to aninsulating coating material from an electronic circuit board for formingon the board an insulating coating layer having high water repellency,quick drying property, high edge coverability and high adhesion.

Further, the present invention relates to an improvement in a method forforming an insulating coating layer for an electronic circuit board andmore particularly to a method for forming an insulating coating layerhaving high water repellency, quick drying property, high edgecoverability and high adhesion for an electronic circuit board withelements mounted thereon.

Prior Art

In a control circuit board for an electronic control unit it isnecessary that an insulating coating layer having water repellency beformed on the surface of the board after mounting of electronic elementsto prevent dew condensation on the board surface which would cause watercoating and short-circuiting. This necessity is high particularly invarious automotive electronic control units because automobiles are usedunder severe outdoor environments.

Such materials as acrylic resins, epoxy resins and silicone resins haveheretofore been used for the formation of insulating coating layers ofthis type. As the method of forming such insulating coating layers therehas been known a dipping method or a spray method, using a relativelyhigh boiling solvent such as toluene or xylene.

On the other hand, a large number of elements are mounted on anelectronic circuit board by passing their lead wire portions throughinsertion holes formed in the board followed by soldering. Fore ends ofthe elements' lead wires are each projecting in the form of a pin asshown in FIG. 1. According to experiments and research done by thepresent inventors, even if a resin solvent for forming a coating layer 6is applied to the pin portion to cover the whole of the pin by a dippingmethod or a spray method, as shown in FIG. 7, the solution applied tothe edge portion of the pin is apt to flow off the edge portion in thecourse of volatilization of the solvent and drying, resulting in such astate as shown in FIG. 8. And it has become clear that the pin top isfinally uncovered as shown in FIG. 9.

The occurrence of such defect may be avoided by thickening the coatinglayer 6, for example, by increasing the number of times the applicationis repeated. However, a thicker coating layer is apt to deteriorateproductivity and induce an internal stress of the coating layer itself,thus causing solder crack at the soldered portion due to heat shock orcrack of the coating layer itself due to heat shrinkage.

SUMMARY OF THE INVENTION Objects of the Invention

The present invention has been accomplished in view of theabove-mentioned background and it is an object of the invention toprovide a coating layer capable of fulfilling an insulating function toa satisfactory extent even for edge portions having electroconductivityof such pin-like projections as referred to above, a method for formingsame, as well as an almost transparent coating layer which is relativelystrong against external physical forces, and a method for forming same.

It is another object of the present invention to provide an insulatingcoating material for forming on an electronic circuit board aninsulating coating layer having high water repellency, quick dryingproperty, high edge coverability and high adhesion, without the need ofpost-treatment, as well as a method for forming the insulating coatinglayer.

Features of the Invention

In order to achieve the above-mentioned objects of the presentinvention, an insulating coating layer for an electronic circuit board,according to the present invention, is characterized by being formed onan electronic circuit board with electronic elements mounted thereon andprovided on the surface thereof with edge portions havingelectroconductivity, the coating thickness at the edge portions and thatat the other portion than the edge portions being almost equal to eachother.

A method for forming an insulating coating layer for an electroniccircuit board, according to the present invention, is characterized inthat a solution of a thermoplastic resin in a low boiling solventcapable of dissolving the thermoplastic resin is sprayed onto thesurface of an electronic circuit board with electronic elements mountedthereon to form an insulating coating layer thereon in which thethickness at edge portions having electroconductivity on the circuitboard and the thickness at the other portion than the edge portions arealmost equal to each other.

An insulating coating layer for an electronic circuit board, accordingto the present invention, is characterized by being formed on anelectronic circuit board with electronic elements mounted thereon andprovided on the surface thereof with edge portions havingelectroconductivity, the coating thickness at the edge portions and thatat the other portion than the edge portions being almost equal to eachother, and the surface of the coating layer being generally flat.

A method for forming an insulating coating layer for an electroniccircuit board, according to the present invention, is characterized byspraying a solution of a thermoplastic resin in a first low boilingsolvent capable of dissolving the thermoplastic resin onto the surfaceof an electronic circuit board with electronic elements mounted thereonto form an insulating coating layer thereon in which the thickness atedge portions having electroconductivity on the circuit board and thethickness at the other portion than the edge portions are almost equalto each other, and subsequently holding the thus-coated circuit board ina vapor atmosphere of a second solvent capable of dissolving the saidresin, for a predetermined time to thereby flatten the surface of thecoating layer.

A method for forming an insulating coating layer for an electroniccircuit board, according to the present invention, is characterized byspraying a solution of a thermoplastic resin in a low boiling solventcapable of dissolving the thermoplastic resin onto the surface of anelectronic circuit board with electronic elements mounted thereon toform an insulating coating layer thereon in which the thickness at edgeportions having electroconductivity on the circuit board and thethickness at the other portion than the edge portions are almost equalto each other, and subsequently heating the coating layer at apredetermined temperature for a predetermined time to soften and meltthe coating layer and to thereby flatten the surface thereof.

Operation

According to the above means, since the insulating coating layer formingsolution is constituted by a low boiling solvent, it has drying propertyand is applicable by a spray method, and when applied onto the board, itimmediately becomes dry or semi-dry. Consequently, a uniform coatinglayer can be formed even at a portion where the solution is apt to flowaway such as an edge portion, thus permitting the edge portion to becoated in an insulated state without the necessity of thickening thecoating layer.

In this state the coating layer is white and dotlike-embossed and has avery high water repellency, but the white color makes it difficult todiscriminate between electronic elements under the coating, and fineconcaves and convexes on the coating surface weaken the coating layeragainst external physical forces such as manual contact. In the presentinvention, however, concaves and convexes on the coating layer surfaceare coating layer against external physical forces such as manualcontact. In the present invention, however, concaves and convexes on thecoating layer surface are melted to flatten to almost the same surfacewithout causing flow of the coating layer, so that the coating layerbecomes transparent in uniform thickness and strong against externalphysical forces.

An insulating coating material for an electronic circuit board,according to the present invention, is characterized in that it is asolution comprising a thermoplastic resin and a mixed solvent capable ofdissolving the thermoplastic resin and that the said mixed solventcomprises a main solvent capable of dissolving the thermoplastic resinand having a boiling point not higher than 100° C. at 760 mmHg and asecondary solvent compatible with and boiling higher than the mainsolvent.

A method for forming an insulating coating layer for an electroniccircuit board, according to the present invention, is characterized byapplying a solution comprising a thermoplastic resin and a mixed solventby spraying onto the surface of an electronic circuit board withelectronic elements mounted thereon, the mixed solvent comprising a mainsolvent capable of dissolving the thermoplastic resin and having aboiling point not higher than 100° C. at 760 mmHg and a secondarysolvent compatible with and boiling higher than the main solvent, andthen drying the said solution to form an insulating coating layer on theboard in which the thickness at edge portions having electroconductivityon the circuit board and the thickness at the other portion than theedge portions are almost equal to each other.

A method for forming an insulating coating layer for an electroniccircuit board, according to the present invention, is characterized bydipping an electronic circuit board with electronic elements mountedthereon into a solution comprising a thermoplastic resin and a mixedsolvent, the mixed solvent comprising a main solvent capable ofdissolving the thermoplastic resin and having a boiling point not higherthan 100° C. at 760 mmHg and a secondary solvent compatible with andboiling higher than the main solvent, then pulling up the electroniccircuit board from the solution to apply the solution onto the surfaceof the circuit board, and subsequently drying the applied solution toform an insulating coating layer in which the thickness at edge portionshaving electroconductivity on the circuit board and the thickness at theother portion than the edge portions are almost equal to each other.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic sectional view showing a structure of aninsulating coating layer according to

example 1 of the present invention;

FIG. 2 is a perspective view showing an electronic circuit board withthe insulating coating layer of FIG. 1 applied thereto;

FIG. 3 is an enlarged sectional view showing a fine surface shape of thecoating layer;

FIG. 4 is an explanatory sectional view showing the state of sprayingthe aerosol of the coating material onto the electronic circuit board;

FIG. 5 is an explanatory sectional view showing a state just beforeadhesion of a coating material onto the electronic circuit board;

FIG. 6 is an explanatory sectional view showing a state afterapplication of the coating material onto the electronic circuit board.

FIG. 7 is an explanatory sectional view showing a state just afterapplication of a conventional insulating coating material;

FIG. 8 is an explanatory sectional view showing a somewhat dropped stateof the coating material;

FIG. 9 is an explanatory sectional view showing a considerably droppedstate of the coating material;

FIG. 10 is a schematic sectional view showing a structure of aninsulating coating layer according to example 2 of the presentinvention;

FIG. 11 is a schematic sectional view showing a structure of aninsulating coating layer formed in example 2;

FIG. 12 is an enlarged sectional view of the insulating coating layershown in FIG. 11;

FIG. 13 is a schematic sectional view showing a structure of aninsulating coating layer resulting from heat treatment of the insulatingcoating layer shown in FIG. 11; and

FIG. 14 is an enlarged sectional view of the insulating coating layershown in FIG. 13.

DETAILED DESCRIPTION OF THE INVENTION

The following concrete examples are given to illustrate the presentinvention in more detail.

EXAMPLE 1

FIG. 2 is a perspective view illustrating an electronic circuit board 1with various electronic elements 3 mounted thereon, in which the numeral2 denotes a substrate (a ceramic substrate will do) formed byimpregnating a base material such as glass cloth, mat, non-woven clothor paper with a phenolic resin, an epoxy resin or a polyester resin in alaminar form, with copper foil bonded to one or both faces thereof. Theelectronic elements 3 are mounted on the substrate 2 by soldering leadwires thereof through holes formed in the substrate, and an electricalconnection is made using a connector 4.

FIG. 1 is a schematic sectional view of the lead wire soldered portionon the back of the electronic circuit board, in which numeral 4 denotesa lead wire serving as a leg of an element such as an integrated circuit(IC) or a resistor, with an electroconductive edge portion being formedat a top corner 4a or at a side corner 4b in the case of the lead wirebeing rectangular in section. Numeral 5 denotes solder and numeral 6adenotes an insulating coating layer according to the present invention,the insulating coating layer 6a being formed throughout the substrate 1,including the lead wire soldered portions. The coating layer 6a isformed, for example, by a perfluoroalkylacryl copolymer resin having ahigh critical surface tension and superior in water repellency. Thesurface thereof is a rough dotlike-embossed surface (frosted glass-likerough surface) having fine concaves and convexes. More particularly, thesurface roughness is approximately 0.1 to 50 μm in terms of H_(MAX)(maximum height) and the pitch between each concave and each convex isin the range of 0.1 to 100 μm. The thickness of the coating layer 6a isabout 1 μm and is almost uniform throughout the entirety thereof. Thatis, the edge portion 4a at the fore end of the lead wire 4 is alsocovered with the coating layer 6a in about the same thickness as at theother portion.

The following description is now provided about how to form the coatinglayer 6a.

The perfluoroalkylacryl copolymer resin was obtained by copolymerizing70-99 wt % of a perfluoroalkylacryl monomer (acrylic acid, methacrylicacid, acrylonitrile or a derivative thereof, or the like having an alkylgroup with side chain all substituted by fluorine) and 1-30 wt % of anacryl monomer as another monomer (acrylic acid, methacrylic acid,acrylonitrile, or a derivative thereof). This resin was dissolved inFreon 113 (a product of Du Pont) as a freon solvent at a concentrationthereof in the range of 0.1 to 15 wt %.

Then, the solution was applied to the substrate surface according to aknown air spray method (aerosolling the solution with a high-pressureair current and spraying the resulting aerosol). The solvent Freon 113(Cl₂ FC-CClF₂, a product of Du Pont) dries quickly because its boilingpoint of 47.6° C. is close to room temperature, so when the solution isapplied to the substrate surface by spraying at a certain distance fromthe substrate, the aerosol adhered to the substrate surface dries andsolidifies by vaporization of the solvent before it forms a liquid layeron the same surface, so that the coating layer formed on the substratesurface is a dotlike-embossed surface as mentioned above, which is likea white frosted glass.

On the other hand, at the fore end portion of the lead wire 4 having anedge portion, almost simultaneouly with or immediately after theadhesion of the aerosol of the resin solution sprayed as in FIG. 4 tothe lead wire surface, the solvent begins to volatilize and dry becauseof its quick drying property, so that a coating layer is formed in thesprayed state of the aerosol in uniform thickness at both the edgeportion 4a and the other portion without the occurrence of suchflowing-off phenomenon as shown in FIGS. 8 and 9. This permits thecoating layer to be formed thin throughout the entirety thereof, so thatthere will not occur an internal stress of the coating layer, crack ofthe soldered portion of each electronic element due to heat shock, orcrack of the coating layer due to heat shrinkage. Consequently,disconnection or short-circuit of the electronic circuit will not occurover a long period of time. Moreover, an economic merit is attainedbecause they may be used a smaller amount of the expensiveperfluoroalkylacryl copolymer as the starting material for the coatinglayer. In the above method, a suitable spray distance is in the range ofabout 20 to about 50 cm although it depends on the amount andconcentration of the solution sprayed.

Reference will be made below to the comparison between the presentinvention and Comparative Examples 1 and 2 with respect to the percentedge coverability and results of an anti-dew condensation test. InComparative Example 1 a coating layer was formed by a dipping methodusing a solution of an acrylic resin as a thermoplastic resin in xylene(b.p. 138°-144° C.) as a solvent, and in Comparative Example 2 a coatinglayer was formed by the same dipping method using a solution of aperfluoroalkylacryl copolymer in Freon 113 (a product of Du Pont). The"percent edge coverability" as referred to herein represents a/b×100 (%)wherein "a" represents the thickness of the coating layer at the edgeportion and "b" represents the coating layer thickness at the otherportion. In the anti-dew condensation test, an electronic circuit fortesting with an insulating coating layer formed thereon was placed in anatmosphere of -30° C. for 30 minutes, then transferred into anatmosphere of 25° C., 90-95% RH and electricity was supplied to theelectronic circuit. This cycle was repeated and whether trouble did notoccur over 30 cycles or more was checked.

                                      TABLE 1                                     __________________________________________________________________________                            Thickness                                                                           Percent                                                                              Anti-dew                                        Resin       How to                                                                             of edge                                                                             edge   condensation                                    material                                                                            Solvent                                                                             apply                                                                              portion                                                                             coverability                                                                         test                                     __________________________________________________________________________    Comparative                                                                          Perfluoro-                                                                          Freon 113                                                                           Dip  1 μm                                                                             50%    x                                        Example 2                                                                            alkylacryl       or less                                                                             or less                                                copolymer                                                              Comparative                                                                          Acrylic                                                                             Xylene                                                                              ↑                                                                            ↑                                                                             ↑                                                                              x                                        Example 1                                                                            resin                                                                  Present                                                                              Perfluoro-                                                                          freon 113                                                                           Spray                                                                              1˜20 μm                                                                    90˜120%                                                                        o                                        Invention                                                                            alkylacryl                                                                    copolymer                                                              __________________________________________________________________________

As is apparent from Table 1, the coating layer of the present inventionis very high in the percent edge coverability and an outstanding effectis recognized also in the operation of the electronic circuit.

According to experiments and research done by the present inventors, asatisfactory insulating and protecting effect for the electronic circuitis attained when the percent edge coverability is not smaller than 60%,which value can be attained very easily in the method of the presentinvention due to the quick drying property explained previously.Moreover, since the insulating coating layer obtained in the aboveworking example employs a perfluoroalkylacryl copolymer having a largecritical surface energy, the resin per se has a high water repellency.Further, its surface is a rough dotlike-embossed surface having fineconcaves and convexes, so even when water drops adhere thereto, the areaof contact with the water drops will be very small, that is, the waterrepellency is improved. Consequently, the edge portion covered generallyuniformly throughout the entirety thereof is improved in its insulatingproperty.

EXAMPLE 2

The coating layer 6a formed in the above working example has a very highwater repellency because of a dotlike-embossed surface thereof asmentioned above, while on the other hand letters printed on electronicelements, etc. are difficult to discriminate because of the white colorof the surface, and the fine concaves and convexes at an upper portionof the coating layer 6a are weak against external physical forces. Inview of this point the following working example has a step of heatingthe dotlike-embossed coating layer 6a, for example, at a temperature of60° to 80° C. for 10 to 20 minutes, as illustrated in FIG. 12, inaddition to the manufacturing process for the coating layer 6a in theabove working example. More particularly, by suitably adjusting theheating temperature and time, mainly the fine concaves and convexesalone on the surface of the dotlike-embossed coating layer 6a can besoftened and melted to flatten the surface and form a coating layer 6bwithout melting the portion on the side of the substrate 2 and lead wire4 of the dotlike-embossed coating layer 6a and hence without causingflow of the coating layer 6a. The heating temperature and time aredetermined according to cases. If the components mounted on thesubstrate 2 are of a high heat resistance, it is possible to form thecoating layer 6b at a higher temperature in a shorter time. As to therange of the heating temperature, in the case of this working example itcan be considered that the softening point of perfluoroalkylacrylmonomer, which is in the range of about 60° to 70° C., is a lower limitand the temperature at which weight loss occurs by heating, which isabout 150 C., is an upper limit.

The following is an explanation of another working example of a methodfor forming the flattened coating layer 6b. Also in this example theprocess up to the formation of the dotlike-embossed coating layer 6a maybe the same as in the previous example, so an explanation thereon willbe omitted. This working example is characterized in that, in order toflatten the surface of the dotlike-embossed coating layer 6a, thesubstrate 2 with the coating layer 6a formed thereon is held in a vaporatmosphere of Freon 113 (a product of Du Pont) as a fluorine-basedsolvent for a predetermined time. Since the coating layer 6a is held insuch vapor atmosphere, not dipped in a solution of Freon 113 (a productof Du Pont), mainly the fine concaves and convexes alone on the surfaceare melted without melting the portion on the side of the substrate 2and lead wire 4 of the dotlike-embossed coating layer 6a, whereby thesurface of the coating layer can be made flat to the same extent as inthe previous working example in which the coating layer surface isheated for the melting. According to an experiment conducted by thepresent inventors, when a substrate 2 of approximately 15 cm by 15 cmwas held in a vapor atmosphere of Freon 113 (a product of Du Pont) for0.1 to 5 seconds, only fine concaves and convexes were melted to asatisfactory extent to afford a transparent coating layer 6.

FIG. 10 is a schematic sectional view of the insulating coating layer 6bobtained by the above working example involving either heating thesubstrate or holding it in a vapor atmosphere of a fluorine-basedsolvent. In the figure, the insulating coating layer 6b is transparent,having a surface roughness not larger than about 20 μm in terms ofH_(MAX) , and its thickness is almost uniform throughout the entiretythereof. On both the edge portion 4a at the fore end of the lead wire 4and the other portion the coating layer is formed in about the samethickness.

In view of the purpose of insulation it is to be understood that therange of almost equal or about the same thickness as referred to hereinincludes a fairly wide range of 50% to 150% in terms of percent edgecoverability.

Examples of resin employable in the present invention include, inaddition to the perfluoroalkylacryl copolymer used in the above workingexample, acrylic resins and other thermoplastic resins having waterrepellency. As the main solvent there may be used any of varioussolvents capable of dissolving the above resins, boiling not higher than80° C. and affording a quick drying property upon spray of the solution.

As the spray there may be adopted airless spray in addition to air sprayusing high pressure air.

As examples of the edge portion having electroconductivity in thepresent invention there are mentioned electroconductive edge portions ofchip elements as well as fore end or side portions of lead wires in theabove working examples.

EXAMPLE 3

The insulating coating material for an electronic circuit boardaccording to the present invention is characterized in that it is asolution comprising a thermoplastic resin and a mixed solvent fordissolving the thermoplastic resin and that the said mixed solventcomprises a main solvent capable of dissolving the thermoplastic resinand having a boiling point not higher than 100° C. at 760 mmHg and asecondary solvent compatible with and boiling higher than the mainsolvent.

The above thermoplastic resin, which is used for the formation of aninsulating coating for an electronic circuit board, is not speciallylimited if only it is dissolved in the mixed solvent as will bedescribed below. Examples are acrylic resins and other resins havingwater repellency, in addition to perfluoroalkylacryl copolymers. Aboveall, perfluoroalkylacryl copolymers are preferred. This is because theycan afford a coating superior in insulating property, water repellency,adhesion and appearance.

The main solvent is of a relatively low boiling point and a relativelylarge volatility. The mixed solvent contains a secondary solventcompatible with and boiling higher than the main solvent. It does notmatter whether the secondary solvent dissolves the resin used or not, ifonly it is compatible with the main solvent. However, in the case ofusing a secondary solvent not dissolving the resin, it is preferablethat its proportion be small or that it be azeotropic with the mainsolvent. This is for preventing whitening of the coating layer withvolatilization of the main solvent. The secondary solvent is foradjusting the drying property of the mixed solvent in a range capable ofmaintaining a high edge coverability and determining a well-balancedsolvent composition range.

It is preferable for the main solvent to have a relatively largevolatility. For example, the main solvent has a boiling point not higherthan 50° C. at 760 mmHg. The main solvent and the secondary solvent aredetermined in correlation with the thermoplastic resin used, but in thecase of using a fluoric resin which is preferred as the thermoplasticresin, it is preferable that the main solvent be a fluorine-basedsolvent and the secondary solvent be a fluorine-based solvent alone or asolvent containing a fluorine-based solvent. Preferably, the secondarysolvent consists of first and second secondary solvents which arecompatible with the main solvent and compatible with each other. As thesecond secondary solvent there may be used a solvent which does notdissolve the resin used. The main solvent-secondary solvent mixing ratiois preferably 0.1-20 parts by weight of the secondary solvent per 100parts by weight of the main solvent. Preferably, the solution as theinsulating coating material has a resin concentration in the range ofabout 0.1 to 15 wt %. In this case, the solution usually has a viscosity(as measured using a B-type viscosimeter) in the range of about 1 cp toabout 50 cp. The resin concentration and the solution viscosity varydepending on the kind of resin used, degree of polymerization, etc.

When the insulating coating material or solution is sprayed onto thesurface of an electronic circuit board with electronic elements mountedthereon, it is preferable that the resin concentration in the solutionjust before or after the application be in the range of 20 to 30 wt %and that the viscosity (as measured using a B-type viscosimeter) of theresin solution just before or after the application be in the range of50 cp to 100 cp. These are for preventing dropping of the solution afterits application onto the substrate surface and thereby forming agenerally uniform coating layer.

As an application method other than the spray method there also may beused a dipping method or a brush coating method for the insulatingcoating material according to the present invention.

Examples of the edge portion having electroconductivity in the presentinvention include fore end and side portions of lead wires as well aselectroconductive edge portions of chip elements.

The method for forming an insulating coating layer for an electroniccircuit board, according to the present invention, is characterized byspraying a solution comprising a thermoplastic resin and a mixed solventonto the surface of an electronic circuit board with electronic elementsmounted thereon, the mixed solvent comprising a main solvent capable ofdissolving the thermoplastic resin and having a boiling point not higherthan 100° C. at 760 mmHg and a secondary solvent compatible with andboiling higher than the main solvent, and then drying the solution toform an insulating coating layer in which the thickness at edge portionshaving electroconductivity on the circuit board and the thickness at theother portion than the edge portions are almost equal to each other.

In this method, where the solution is applied by spraying onto thesurface of the electronic circuit board with electronic elements mountedthereon, it is preferable that the viscosity (as measured using a B-typeviscosimeter) of the solution just before or after the application be inthe range of about 50 cp to about 100 cp.

The thermoplastic resin and the mixed solvent used in this method may bethe same as those explained previously. In this method, moreover, theresin concentration in the solution just after application onto thesurface of the electronic circuit board may be in the range of 15 to 50wt %. The application of the solution may be done using a known airspray method (aerosolling the solution using a high pressure air currentand spraying the resultant aerosol). In place of the high pressure airspray method there may be adopted an airless spray method. The spraydistance is preferably in the range of about 20 to about 50 cm althoughit depends on the amount and concentration of the solution sprayed.

The method for forming the insulating coating layer for an electroniccircuit board is not limited to the spray method. There may be adopted adipping method. According to a dipping method, an electronic circuitboard is dipped into such a predetermined solution as mentioned above,then the circuit board is pulled up from the solution to apply thesolution onto the surface of the circuit board and the solution is thendried to form a predetermined coating. In this case, the circuit boardmay be pulled up directly from the solution, or a tank containing thesolution may be pulled down by means of a lifter or the like to pull upthe circuit board from the solution in a relative manner.

It is preferable that the viscosity of the solution just after pullingup the circuit board from the solution be in the range of 50 to 100 cp.This is for preventing the solution from dropping just after pulling upfrom the solution to thereby make the coating thickness at edge portionson the circuit board and the coating thickness at the other portion thanthe edge portions almost equal to each other.

The present invention will be described below on the basis of concreteworking examples. In the method used in the above Example 2 it isnecessary to perform a post-treatment for flattening the coating. On theother hand, in Example 3, the above object can be attained withoutrequiring such post-treatment.

As the thermoplastic resin there was used a perfluoroalkylacrylcopolymer resin which had been obtained by copolymerizing 70-99 wt % ofa perfluoroalkylacryl monomer (acrylic acid, methacrylic acid,acrylonitrile or a derivative thereof having an alkyl group with sidechain all substituted by fluorine) and 1-30 wt % of an acryl monomer asanother monomer (acrylic acid, methacrylic acid, acrylonitrile, or aderivative thereof).

As the main solvent there was used Freon 113 (a product of Du Pont)having a good compatibility with the above resin and a very superiorquick drying property. As the secondary solvent there may be used any ofthe following three solvents. The first is a solvent (especially afluorine-based solvent) boiling higher than Freon 113 (a poduct of DuPont) and capable of dissolving the above fluoric resin. The second is asolvent (especially a solvent azeotropic with Freon 113 (a product of DuPont) and not dissolving the fluoric resin) having a high boiling pointand compatible with Freon 113 (a product of Du Pont). The third one is abinary solvent consisting of the first and second solvents.

According to experiments conducted by the present inventors, m-xylenehexafluoride (m-XHF, boiling point 110° C.) is effective as a secondarysolvent capable of dissolving the above resin. This is because m-XHF issuperior not only in dissolving fluoric resins but also in suppressingthe volatility of the main solvent moderately. High boiling organicsolvents compatible with Freon 113 (a product of Du Pont) are shown inTable 2. Various such solvents are existent as shown therein, butethanol (b.p. 67° C.) is considered suitable in view of its lowtoxicity, low cost, suitable boiling point, azeotropic characteristicand suitability for electronic components.

The following resin solutions were prepared in consideration of theabove.

                  TABLE 2                                                         ______________________________________                                        Dissolve Dissolve                 Dissolve                                    completely                                                                             very well    Dissolve well                                                                             a little                                    ______________________________________                                        Acetone  Azobenzene   Acetanilide Acetamide                                   Carbon   Camphor      Benzoic acid                                                                              Gelatin                                     tetrachloride                                                                 Benzene  Benzophenone Benzyl      Anthracene                                  Chloroform                                                                             Cocoa butter Diphenyl    Lactic acid                                 Ether    Cottonseed oil                                                                             Diphenylcar-                                                                              Iodoform                                                          binol                                                   Methanol Naphthalene  Ester gum   Phenol                                      Ethanol  Thiobromine  Fthyl bromide                                                                             Salicylic acid                              Hexane   Thymol       Hexachloro- Sulfonal                                                          ethane                                                  Kerosene Tribromophenol                                                                             Phthalic    Tartaric acid                                                     anhydride                                               Mineral oil           Stearic acid                                                                              Urea                                        ______________________________________                                    

                                      TABLE 3                                     __________________________________________________________________________                        Quick                                                            Coating      drying                                                                              Edge   Water                                               Appearance                                                                           Adhesion                                                                            property                                                                            coverability                                                                         repellency                                   __________________________________________________________________________    Run 1  Good   Very good                                                                           Somewhat                                                                            Somewhat                                                                             Somewhat                                                         slow  poor   poor                                         Run 2  Good   Good  Good  Good   Good                                         Run 3  Good   Very good                                                                           Good  Good   Good                                         Comparative                                                                          White  Somewhat                                                                            Good  Good   Very good                                    Run 1         poor                                                            __________________________________________________________________________

The resin concentration was adjusted to about 2 wt % using as a solventFreon 113 (a product of Du Pont) and m-XHF in Run 1, Freon 113 (aproduct of Du Pont) and ethanol in Run 2, Freon 113, mXHF and ethanol inRun 3, and Freon 113 (a product of Du Pont) alone in Comparative Run 1.

The above resin solutions were sprayed under the following conditions.Results are as set out in Table 3.

(Spray Conditions)

Airless spray (a spraying method by which an oblong sprayed portion isobtained)

Spray distance: 20-30 cm

Pattern width: 15-20 cm

Moving rate of the gun or substrate: 100 cm/min.

In Run 1, at an optimum ratio of the main solvent to the secondarysolvent in the range of 20/1 to 5/1, there was exhibited a very goodresult in the improvement of adhesion although the quick drying propertyand the edge coverability were somewhat inferior. That is presumed to bebecause the volatilization of the mixed solvent was suppressed to someextent due to a relatively high boiling point (110° C.) of the secondarysolvent in comparison with the main solvent and also because thesecondary solvent dissolved the fluoric resin.

In Run 2, since the boiling point 67° C. of the secondary solvent is ofa medium degree and an azeotropic mixture with Freon 113 (a product ofDu Pont) is constituted, there are attained easy handleability, quickdrying property and good edge coverability. However, the improvement ofadhesion is somewhat inferior to that in Run 1 because the secondarysolvent in Run 2 does not have a dissolving power for the resin. In Run3, possessing both the merit of Run 1 and that of Run 2, the bestperformance was exhibited in an appropriate balance of the mixedmain-secondary solvent system.

The amount of the solvent components to be added varies according toamount of resin or spray conditions, etc., so it is necessary todetermine it case by case. As examples of secondary solvent components:acetone, hexane, methanol, other alcohols, xylene, ether and benzene.Also effective as a fluorine-based solvent is tetrachlorodifluoroethane.

Table 4 shows the results of an anti-dew condensation test forelectronic circuits which have been subjected to spray treatments (airspray or airless spray method) using the above coating agents. InComparative Run 2 there was performed a dipping treatment using anacrylic resin as a thermoplastic resin and xylene as a mixed solvent,not containing a secondary solvent. In the anti-dew condensation test,an electronic circuit for test having an insulating coating layer formedthereon was allowed to stand in an atmosphere of -30° C. for 30 minutesand thereafter transferred into an atmosphere of 25° C., 90-95% RH, thenthe electronic circuit was supplied with electricity followed bychecking, and this cycle was repeated. The maximum number of cycles wasset to ten. A look at the results shows that in Runs 2 and 3 there wasattained an extremely good anti-dew condensation performance withoutcausing deterioration in the supply of electricity even after tencycles.

                  TABLE 4                                                         ______________________________________                                        Number of                                                                     cycles     2       4     6     7   8     9     10                             ______________________________________                                        Run 1      o       o     o         o     o     x                              Run 2      o       o     o         o           o                              Run 3      o       o     o         o           o                              Comparative                                                                              o       o     o         o           o                              Run 1                                                                         Comparative                                                                              o       o     o     x                                              Run 2                                                                         ______________________________________                                    

Particularly, according to experiments conducted by the presentinventors, an extremely good result was obtained by using a resinsolution which had been prepared by adding 3-8 parts by weight of m-XHFand 2-5 parts by weight of ethanol to a resin solution consisting of 2parts by weight of solid resin and 98 parts by weight of Freon 113 (aproduct of Du Pont). The use of this coating material was also effectivein preventing bubbling during not only spray treatment but also dippingtreatment.

(Effect of the Invention)

According to the present invention, as set forth hereinabove, on anelectronic circuit board having electroconductive edge portions andprojections there is formed an insulating coating layer in which thethickness at the edge portions and that at the portion other than theedge portions are almost equal to each other, whereby it is madepossible to provide an electronic circuit board having a high insulatingproperty against dew condensation.

Moreover, by flattening fine concaves and convexes formed on the coatinglayer surface, the resistance to external physical forces can beenhanced and it is possible to render the said surface transparent, thusresulting in easy differentiation between Nos. of electronic elements,resister color, etc. and resistance can be obtained against the contactof operator's hand, jig, etc.

The mixed solvent used in the insulating coating material for anelectronic circuit board is characterized by comprising a main solventcapable of dissolving a thermoplastic resin and having a boiling pointnot higher than 100° C. at 760 mmHg and a secondary solvent compatiblewith and boiling higher than the main solvent, the main solvent having arelatively large volatility and the secondary solvent suppressing thevolatility of the main solvent moderately. Therefore, when suchinsulating coating material is applied to the circuit board surface by aspray method or a dipping method, especially by the former, it ispossible to attain through, for example, a simple spray treatment aninsulating coating layer of high functions having high water repellency,quick drying property, high edge coverability and high adhesion.Further, coupled with the high water repelling effect of the resinitself used, there is attained an outstanding water repelling effectespecially in the case of using a fluorine-contained resin; that is, apractical effect is exhibited as an excellent moisture-proof coatingmaterial for an electronic circuit board.

Additionally, in the method for forming such insulating coating layeraccording to the present invention, when the above insulating coatingmaterial is applied by spraying or dipping onto the surface of anelectronic circuit board, the coating solution just before or after theapplication scarcely drops, so upon drying of the solution the coatingthickness at edge portions and that at the other portion become almostequal to each other, thus permitting the pin edge portions to be coatedto a sufficient extent.

Besides, when the insulating coating material of the present inventionis used for forming the insulating coating layer, the quick dryingproperty thereof is suppressed by the secondary solvent, so that anappropriate drying speed is obtained, affording a highly transparentcoating appearance.

Having now fully described the invention, it will be apparent to one ofordinary skill in the art that many changes and modifications can bemade thereto without departing from the spirit or scope of the inventionas set forth herein.

I claim:
 1. An insulating coating material for an electronic circuitboard consisting essentially of a fluorinated thermoplastic resin and amixed solvent for dissolving said fluorinated thermoplastic resin, saidmixed solvent comprising a main solvent and a secondary solvent, saidmain solvent comprising 1,1,2-trichloro-1,2,2-trifluoroethane (bp. 47°C.), and said secondary solvent being compatible with and having ahigher boiling point than said main solvent.
 2. An insulating coatingmaterial for an electronic circuit board according to claim 1, whereinsaid secondary solvent has a boiling point greater than 100° C.
 3. Aninsulating coating material for an electronic circuit board according toclaim 1, wherein said secondary solvent is m-xylene hexafluoride (bp.113° C.).
 4. An insulating coating material for an electronic circuitboard according to claim 3, wherein the content of said secondarysolvent consists of 0.1 to 20 weight parts per 100 weight parts of saidmain solvent, and the content of said fluorinated thermoplastic resin isin the range of from 0.1 to 15 wt % of said insulating coating material.5. An insulating coating material for an electronic circuit boardaccording to claim 3, wherein said fluorinated thermoplastic resin is aperfluoroalkylacryl copolymer resin.
 6. An insulating coating materialfor an electronic circuit board according to claim 5, wherein saidperfluoroalkylacryl copolymer resin comprises of from 70 to 99 wt % ofperfluoroalkylacryl monomer, and from 1 to 30 wt % of an acryl monomer.7. An insulating coating material for an electronic circuit boardaccording to claim 6, wherein said perfluoroalkylacryl monomer isselected from the group consisting of perfluoroacrylic acid,perfluoromethacrylic acid, perfluoroacrylonitrile and derivativesthereof, and said acryl monomer is selected from the group consisting ofacrylic acid, methacrylic acid, acrylonitrile and derivatives thereof.8. An insulating coating material for an electronic circuit boardaccording to claim 3, wherein said coating material has a viscosity inthe range of from about 1 cp to about 50 cp.
 9. An insulating coatingmaterial for an electronic circuit board according to claim 3, whereinsaid secondary solvent is capable of dissolving said fluorinatedthermoplastic resin.
 10. An insulating coating material for anelectronic circuit board according to claim 3, wherein said secondarysolvent is not capable of dissolving said fluorinated thermoplasticresin.
 11. An insulating coating material for an electronic circuitboard according to claim 1, wherein said secondary solvent is ethanol(bp. 67° C.).
 12. An insulating coating material for an electroniccircuit board consisting essentially of a mixture of a fluroinatedthermoplastic resin and a mixed solvent for dissolving said fluorinatedthermoplastic resin, said mixed solvent comprising a main solvent and afirst and a second secondary solvent, said main solvent comprising1,1,2-trichloro-1,2,2-trifluoroethane (bp. 47° C.), said first secondarysolvent being compatible with and having a higher boiling point thansaid main solvent, and said first secondary solvent being capable ofdissolving said fluorinated thermoplastic resin, said second secondarysolvent being compatible with and having a higher boiling point thansaid main solvent, and said second secondary solvent not being capableof dissolving said fluorinated thermoplastic resin.
 13. An insulatingcoating material for an electronic circuit board according to claim 12,wherein said first secondary solvent is m-xylene hexafluoride (bp. 113°C.) and said second secondary solvent is ethanol (bp. 67° C.).
 14. Aninsulating coating material for an electronic circuit board consistingessentially of a fluorinated thermoplastic resin and a mixed solvent fordissolving said fluorinated thermoplastic resin, said mixed solventcomprising a main and a secondary solvent, said main solvent comprising1,1,2-trichloro-1,2,2-trifluoroethane (bp. 47° C.), said secondarysolvent being compatible with and having a higher boiling point thansaid main solvent, and said secondary solvent being capable ofdissolving said fluorinated thermoplastic resin.
 15. An insulatingcoating material for an electronic circuit board consisting essentiallyof a mixture of a fluorinated thermoplastic resin and a mixed solventfor dissolving said fluorinated thermoplastic resin, said mixed solventcomprising of a main solvent and a secondary solvent, said main solventcomprising 1,1,2-trichloro-1,2,2-trifluoroethane (bp. 47° C.), and saidsecondary solvent being compatible with and having a higher boilingpoint than said main solvent, said secondary solvent not being capableof dissolving said fluorinated thermoplastic resin.